Complexes derived from the reactions of diphenylphosphinoacetic acid. Part 3. Dimetal complexes from [Mo2Cl8]4- and [Re2Cl8]2-

Article abstract of DOI:10.1016/S0020-1693(02)01142-8

Diphenylphosphinoacetic acid (1) reacts with K₄Mo₂Cl₈ in refluxing methanol to afford the centrosymmetric dimolybdenum(II) complex Mo₂Cl₄(μ-Ph₂PCH₂CO ₂Me)₂ (2) that contains two trans P,O bridging phosphine-ester ligands, an eclipsed rotational geometry, and a Mo-Mo quadruple bond distance of 2.1450(8) A?. The corresponding reactions of 1 with (n-Bu₄N)₂Re₂Cl₈ in refluxing methanol and ethanol yield the paramagnetic, dirhenium(III,II) complexes Re₂Cl₅(Ph₂PCH₂CO₂R) ₃ (R=Me (3) or Et (4)) in which the phosphine-ester ligands are P-bound. A crystal structure determination of 3 shows it to have the 1,3,6-structure with a Re-Re bond length of 2.2318(3)A? in accord with a Re-Re bond order of 3.5.

Full text of DOI:10.1016/S0020-1693(02)01142-8

Inorganica Chimica Acta 342 (2003) 267ꢂ271
/
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Complexes derived from the reactions of diphenylphosphinoacetic
acid. Part 31. Dimetal complexes from [Mo2Cl8]4ꢀ and [Re2Cl8]2ꢀ
Shan-Ming Kuang a, Dennis A. Edwards b, Phillip E. Fanwick a, Richard A. Walton a,ꢁ
a Department of Chemistry, Purdue University, 1393 Brown Building, West Lafayette, IN 47907-1393, USA
b School of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
Received 17 April 2002; accepted 20 May 2002
Abstract
Diphenylphosphinoacetic acid (1) reacts with K4Mo2Cl8 in refluxing methanol to afford the centrosymmetric dimolybdenum(II)
complex Mo2Cl4(m-Ph2PCH2CO2Me)2 (2) that contains two trans P,O bridging phosphineꢂ
/
ester ligands, an eclipsed rotational
Mo quadruple bond distance of 2.1450(8) A. The corresponding reactions of 1 with (n-Bu4N)2Re2Cl8 in
refluxing methanol and ethanol yield the paramagnetic, dirhenium(III,II) complexes Re2Cl5(Ph2PCH2CO2R)3 (RꢃMe (3) or Et (4))
in which the phosphineꢂester ligands are P-bound. A crystal structure determination of 3 shows it to have the 1,3,6-structure with a
ReÃRe bond length of 2.2318(3)A in accord with a ReÃ
# 2002 Elsevier Science B.V. All rights reserved.
˚
geometry, and a MoÃ
/
/
/
˚
/
/Re bond order of 3.5.
Keywords: Crystal structures; Diphenylphosphinoacetic acid; Phosphine-ester ligands; Dimolybdenum(II) complexes; Dirhenium(III,II) complexes;
Metalꢂmetal multiple bonds
/
1. Introduction
viously been used quite extensively in mononuclear
transition metal chemistry [5ꢂ17]. Our original intent
/
One focus of our interest in incorporating multiply
bonded dirhenium units into homometallic assemblies
has been the use of bridging dicarboxylate ligands to
generate ‘dimers-of-dimers’, supramolecular triangles,
oligomers, and hydrogen-bonded polymeric chains in
which paramagnetic [Re2]5ꢄ and/or diamagnetic
had been to use the resulting products, which we
expected would contain free P donors or carboxylic
acid groups, to complex other metal centers and thereby
generate new types of mixed-metal assemblies in which
multiply bonded dimetal units would be present. This
strategy has been used previously to access heterome-
tallic complexes in which the (h5-C5H5)2Ti(IV) unit is
linked to Pd(II), Pt(II), Cu(I), Ag(I) and Au(I) centers
[17]. Instead, our work led to some unanticipated
results, details of which are now described.
[Re2]4ꢄ cores are present [1ꢂ
3]. More recently, the
/
strategy of using monocarboxylic acid ligands that also
contain other accessible donor sites, such as 4-pyridine-
carboxylic acid (isonicotinic acid), to obtain heterome-
tallic assemblies has been successfully used to form
molecular squares, in which there are alternating multi-
ply bonded dirhenium [Re(II)]2 and mononuclear Pt(II)
units at the corners [4].
2. Experimental
In the present report, we describe reactions utilizing
diphenylphosphinoacetic acid (1) in which we attempted
to form multiply bonded dimetal complexes that contain
the diphenylphosphinoacetate ligand, which has pre-
2.1. Starting materials and reaction procedures
The starting materials K4Mo2Cl8 [18] and (n-
Bu4N)2Re2Cl8 [19] were prepared by the standard
literature methods. Diphenylphosphinoacetic acid (1)
was prepared by a procedure different from that which
has typically been used [5]. A 2.5 M hexane solution of
n-BuLi (22 ml, 55 mmol) was added dropwise to a
ꢁ Corresponding author. Tel.: ꢄ
/
1-765-494 5464; fax: ꢄ1-765-494
/
0239
E-mail address: rawalton@purdue.edu (R.A. Walton).
1
Part 1 is ref [16]; Part 2 is ref [17].
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 4 2 - 8

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S.-M. Kuang et al. / Inorganica Chimica Acta 342 (2003) 267ꢂ271
/
solution of Ph2PH (9.30 g, 50 mmol) in THF (50 ml) at
0 8C. This mixture was stirred at 25 8C for 1 h and then
3.68; Cl, 12.69. Calc. for C48H51Cl5O6P3Re2: C, 42.19;
H, 3.76; Cl, 12.97%.
added dropwise with stirring to
a solution of
ClCH2CO2Na (5.60 g, 50 mmol) in THF at room
temperature (r.t.). The solvent was evaporated and the
residue dissolved in 100 ml of water, the extract filtered,
2.5. X-ray crystallography
Single crystals with the compositions Mo2Cl4(m-
Ph2PCH2CO2Me)2 (2) and Re2Cl5(Ph2PCH2CO2Me)3
(3), were obtained as described by use of the individual
synthetic procedures outlined in Sections 2.2 and 2.3.
and the filtrate washed with diethyl ether (2ꢅ10 ml)
/
and the washings discarded. The careful addition of 2M
HCl to this aqueous solution afforded a white crystalline
solid that was washed with methanol and hexane. Yield,
The data were collected at 173(9
170(91)K for 3. All measurements were carried out on a
Nonius Kappa CCD diffractometer with graphite-
/
1) K for 2 and
1
11.1 g (91%). The H and 31P{1H} NMR spectral data
/
of this product were identical to the literature data
[5,16]. All other reagents and solvents were obtained
from commercial sources. Solvents were dried and
degassed by standard methods and distilled prior to
use. Reactions were carried out under an atmosphere of
dry dinitrogen.
˚
monochromated Mo Ka radiation (lꢃ0.71073 A).
/
Lorentz and polarization corrections were applied to
the data sets. The important crystallographic data are
given in Table 1.
The structure of 2 was solved using the structure
solution program PATTY in DIRDIF92 [20] while DIR-
DIF99 [21] was used for 3. The remaining atoms were
located in succeeding difference Fourier syntheses.
Hydrogen atoms were placed in calculated positions
2.2. Synthesis of Mo2Cl4(m-Ph2PCH2CO2Me)2 (2)
A mixture of K4Mo2Cl8 (63 mg, 0.10 mmol) and 1
(110 mg, 0.45 mmol) in 40 ml of methanol was refluxed
for 2 days. The mixture of blue crystals and red powder
was filtered off and the solid products washed with
dimethylformamide to dissolve the red powder and leave
the blue crystals of 2; yield 15 mg (18%). Anal. Found:
C, 41.83; H, 3.56. Calc for C30H30Cl4Mo2O4P2: C,
42.38; H, 3.56%. This product, which was identified by
X-ray crystallography, was insoluble in common polar
and non-polar solvents. The red colored dimethylfor-
mamide extract was treated with an excess of diethyl
ether to precipitate a red solid; yield 48 mg. Anal.
Found: C, 37.70; H, 3.46%. This solid was soluble only
in DMF and DMSO; 31P{1H} NMR spectrum (in
according to idealized geometries with U(H)ꢃ1.3
/
Ueq(C). They were included in the refinement but
constrained to ride on the atom to which they are
bonded. An empirical absorption correction using
SCALEPACK [22] was applied in both cases. The final
refinements were performed by the use of the program
SHELXL-97 [23]. All non- hydrogen atoms were refined
with anisotropic thermal parameters unless indicated
otherwise. Crystallographic drawings were done using
the program ORTEP [24].
The structure solutions and refinements proceeded
without major problem. During the latter stages of the
refinement of the dirhenium complex 3, residual electron
(CD3)2SO) dꢃ
/
ꢀ11.3(s, br). The identity of this product
/
was not established, but it does not appear to be closely
related to 2 based upon a comparison of their IR spectra
(KBr pellets).
Table 1
Crystallographic data for Mo2Cl4(m-Ph2PCH2CO2Me)2 (2) and Re2-
Cl5(Ph2PCH2CO2Me)3 (3)
2
3
2.3. Synthesis of Re2Cl5(Ph2PCH2CO2Me)3 (3)
Empirical formula
Formula weight
Space group (No. 14)
C
30H30Cl4Mo2O4P2
C45H45Cl5O6P3Re2
1324.37
P21/n
850.21
P21/n
10.3764(5)
8.6362(7)
18.4587(14)
90
A mixture of (n-Bu4N)2Re2Cl8 (114 mg, 0.10 mmol)
and 1 (100 mg, 0.40 mmol) was refluxed in 70 ml of
methanol for 24 h, then allowed to cool to r.t., and the
green crystals of 3 filtered off and washed with methanol
˚
a (A)
11.27010(10)
22.0133(3)
20.5363(3)
90
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
V (A )
Z
(2ꢅ
/
5 ml) and diethyl ether (2ꢅ5 ml); yield 99 mg
/
95.818(4)
90
97.6124(6)
90
(75%). Anal. Found: C, 40.62; H, 3.31. Calc for
C45H45Cl5O6P3Re2: C, 40.81; H, 3.42%.
3
˚
1645.6(3)
2
1.716
5049.99(11)
4
1.742
rcalcd (g cmꢀ3
)
m (Mo Ka) (mmꢀ1
)
1.201
0.037
5.193
0.043
2.4. Synthesis of Re2Cl5(Ph2PCH2CO2Et)3 (4)
R(Fo)a
2 b
Rw(Fo
)
0.075
1.012
0.089
1.152
The reaction of (n-Bu4N)2Re2Cl8 (118 mg, 0.10 mmol)
and 1 (110 mg, 0.45 mmol) in 40 ml of refluxing ethanol
for periods of up to 2 days gave 4 as green flakey
crystals; yield 91 mg (71%). Anal. Found: C, 41.91; H,
Goodness-of-fit
RꢃajjFojꢀjFcjj/ajFoj with Fo2ꢀ2s(Fo2).
a
b
2
2
2 2 1/2
Rwꢃ[aw(jFo jꢀjFc j)2/awjFo j ]
.

S.-M. Kuang et al. / Inorganica Chimica Acta 342 (2003) 267ꢂ
/271
269
6 e Aꢀ3) was found close to the multiply
bonded dirhenium unit and orthogonal to it. The
distance between these peaks was approximately 2.20
structure determination showed the blue crystals to be
those of the dimolybdenum(II) complex Mo2Cl4(m-
Ph2PCH2CO2Me)2 (2) which is formed by the esterifica-
tion of the diphenylphosphinoacetic acid in the metha-
nol solvent and the coordination of this neutral ligand in
a bridging mode through its phosphine and carbonyl
groups (Fig. 1). The important bond distances and bond
angles for this complex are given in the caption to Fig. 1.
The molecule lies on a crystallographic inversion center
and retains a rigorously eclipsed rotational geometry
like the parent [Mo2Cl8]4ꢀ anion [27]. The conforma-
tional demands of the puckered
˚
density (ꢁ
/
5ꢂ
/
˚
A and they are almost certainly associated with the type
of disorder commonly encountered in dimetal complexes
that contain a metal-metal multiple bond and an
eclipsed M2L8 geometry [25], in which two approxi-
mately orthogonal dimetal sets share the same set of
ligand atoms. Among compounds that show this
disorder are several of the type Re2Cl5(PR3)3 [26]. In
the present case, inclusion of this very minor form in the
refinement gave multiplicities for the major and minor
forms of 0.968 and 0.032, respectively, and led to a
reduction in the final R indices. The largest peak in the
final difference Fouriers of 2 and 3 had a height of 0.51
and 1.97 e Aꢀ3, respectively.
˚
ring do not enforce a staggered or partially staggered
geometry as occurs, for example, in the case of b-
2.6. Physical measurements
Mo2Cl4(dppe)2, where dppeꢃPh2PCH2CH2PPh2, for
/
which the average torsional (twist) angle is approxi-
mately 308 [27]. Consequently, the MoÃMo distance of
2.1450(8)A in 2 is shorter than that in b-Mo2Cl4(dppe)2
Infrared spectra were recorded as KBr pellets on a
1
/
˚
˚
Perkinꢂ
/Elmer 2000 FT-IR spectrometer, while H and
31P{1H} NMR spectra were obtained on a Varian
INOVA 300 spectrometer. Proton resonances were
referenced internally to the residual protons in the
incompletely deuteriated solvent. The 31P{1H} spectra
were recorded at 121.6 MHz, with 85% H3PO4 as an
external standard. The X-band ESR spectrum of
compound 3 was recorded at 150 K on a Bruker ESP
300 instrument. Electrochemical measurements were
carried out with the use of a BAS Inc. Model CV-27
instrument in conjunction with a BAS Model RXY
recorder and were recorded on dichloromethane solu-
tions that contained 0.1 M tetra-n-butylammonium
hexafluorophosphate (TBAH) as supporting electrolyte.
(2.183(3)A) [28], in which the d component of the MoÃ
/
Mo quadruple bond has been weakened, but is very
˚
similar to that in a-Mo2Cl4(dppe)2 (2.140(2)A) [29], in
which the dppe ligands are chelating and the molecule
has an eclipsed geometry. The IR spectrum of 2 (KBr
pellet) shows n(COO) modes of the methyl ester group
at 1630(s) and 1341(m-s) cmꢀ1
.
In the analogous reaction between the dirhenium(III)
complex (n-Bu4N)2Re2Cl8 and 1, the use of acetone or
E1/2 values, determined as (Ep,a
to the silverꢂsilver chloride (Agꢂ
25 8C and were uncorrected for junction potentials.
Under our experimental conditions E1/2 0.47 V
versus AgꢂAgCl for the ferroceniumꢂferrocene couple.
ꢄ
/
Ep,c)/2, were referenced
/
/AgCl) electrode at
ꢃ
/
ꢄ
/
/
/
Elemental microanalyses were performed by Dr. H. D.
Lee of the Purdue University Microanalytical Labora-
tory.
3. Results and discussion
Fig. 1. ORTEP [24] representation of the structure of the dimolybde-
num(II) complex Mo2Cl4(m-Ph2PCH2CO2Me)2 (2). Thermal ellipsoids
are drawn at the 50% probability level except for the carbon atoms of
the phenyl rings which are circles of arbitrary radius. The molecule
The dimolybdenum(II) complex K4Mo2Cl8 failed
react
to
with
diphenylphosphinoacetic
acid
(Ph2PCH2CO2H, 1) at room temperature in a variety
of polar and non-polar solvents, and in refluxing
acetone, dichloromethane or THF. However, in reflux-
ing methanol a slow reaction ensues to afford a mixture
of red powder and blue crystals. The blue crystals were
insoluble in polar and non-polar solvents and could
easily be separated from the unidentified DMF-soluble
red material (Section 2.3). A single crystal X-ray
contains an inversion center at the mid-point of the MoÃMo bond; the
/
˚
unlabeled Mo atom is Mo?. Selected bond distances (A) and bond
angles (8) are as follows: MoÃMo? 2.1450(8), MoÃO(21) 2.198(3),
MoÃCl(1) 2.3914(11), MoÃCl(2) 2.4006(12), C(2)ÃO(3) 1.313(5),
C(2)ÃO(21) 1.234(5); Mo?ÃMoÃO(21) 110.48(8), Mo?ÃMoÃCl(1)
105.62(5), Mo?ÃMoÃCl(2) 108.57(4), Mo?ÃMoÃP(1) 92.26(3), MoÃ
O(21)ÃC(2) 130.2(3), O(21)ÃC(2)ÃC(1) 123.1(4), C(2)ÃC(1)ÃP(1)
107.0(3), C(1)ÃP(1)ÃMo 109.43(15).
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/

270
S.-M. Kuang et al. / Inorganica Chimica Acta 342 (2003) 267ꢂ271
/
dichloromethane as solvents failed to afford pure
products. However, we obtained crystals of the pure
dirhenium(III,II) complex Re2Cl5(Ph2PCH2CO2Me)3
(3) from hot methanol while the ethyl ester derivative
Re2Cl5(Ph2PCH2CO2Et)3 (4) was obtained with the use
of hot ethanol as the reaction solvent. Unlike, the
reaction with K4Mo2Cl8, the reactions involving (n-
Bu4N)2Re2Cl8 lead to a one-electron reduction of the
dimetal core and the resulting complexes 3 and 4 contain
The identity of compound 3 as a complex with a
paramagnetic [Re2]5ꢄ core was confirmed by the pre-
sence of a band at 1410 nm in its electronic absorption
spectrum (Nujol mull); this is assigned to the d ꢂ
/
dꢁ
transition of a dirhenium(III,II) complex [32ꢂ34]. Its
/
ESR spectrum (powder and CH2Cl2-toluene glass, 150
K) shows the typical complex hyperfine splitting pattern
centered close to 2.7 kG (gꢃ2.5) that is seen for other
/
Re2Cl5(PR3)3 complexes [26,31b] and closely related
species [32].
The cyclic voltammetric properties of 3 and 4 are very
similar, each showing reversible processes that corre-
spond to one-electron oxidation (E1/2(ox)) and a one-
electron reduction (E1/2(red)) of the bulk complexes.
monodentate P-bound PPh2(CH2CO2R) (RꢃMe or Et)
/
ligands as shown by a single crystal X-ray structure
determination of 3 (Fig. 2). The important bond
distances and bond angles for 3 are given in the caption
to Fig. 2. This structure is of the previously known
Re2Cl5(PR3)3 type [30], several examples of which have
now been described by Cotton and co-workers [26,31];
these can exist in 1,3,6- and 1,2,7- isomeric forms.
These half-wave potentials are at ꢄ
versus AgꢂAgCl for 3 and at ꢄ0.61V and ꢀ
versus AgꢂAgCl for 4, values which are similar to those
/
0.66V and ꢀ
/
0.46 V
/
/
/
0.44V
/
reported previously for other Re2Cl5(PR3)3 complexes
[26,31,32]. The uncomplexed ester groups in 3 and 4
exhibit n(COO) modes in their IR spectra at 1734(s) and
1266(s) cmꢀ1, and 1725(s) and 1264(s) cmꢀ1, respec-
tively.
Compound 3 is the more commonly encountered
˚
Re bond length of 2.2318(3)A
1,3,6-isomer, and its ReÃ
/
is a little longer than the ReÃ
/
Re distances reported for
the previously characterized trialkylphosphine PR3
(Rꢃ
/
Me, Et or Prn) and PMe2Ph derivatives that have
˚
2.222 A) [26,31]. This
the 1,3,6-structure (range 2.211ꢂ
/
complex has the expected eclipsed rotational geometry
with values for the torsional angles Cl(11)ÃRe(1)Ã
Re(2)ÃCl(21), P(2)ÃRe(1)ÃRe(2)ÃCl(22), Cl(12)Ã
Re(1)ÃRe(2)ÃP(3) and P(1)ÃRe(1)ÃRe(2)ÃCl(23) of
0.8, ꢀ1.4, 1.1 and 3.48, respectively.
4. Concluding remarks
/
/
/
/
/
/
/
The reactions of diphenylphosphinoacetic acid with
the quadruply bonded dimolybdenum(II) and dir-
henium(III) complexes K4Mo2Cl8 and (n-Bu4N)2Re2Cl8
in refluxing alcohol solvents afford the phosphine-ester
/
/
/
/
/
/
ligands Ph2PCH2CO2R (RꢃMe or Et) which display
/
quite different bonding modes in the resulting dimetal
complexes. While K4Mo2Cl8 affords the centrosym-
metric, non-redox product Mo2Cl4(m-Ph2PCH2-
CO2Me)2, the [Re2Cl8]2ꢀ anion is reduced in boiling
alcohol solvents to produce the paramagnetic complexes
1,3,6-Re2Cl5(Ph2PCH2CO2R) (RꢃMe or Et) in which
/
the phosphine-ester ligand is exclusively P-bound. These
are the first multiply bonded dimetal complexes that
?
contain ligands of the type R2PCH2CO2R.
5. Supplementary material
Tables giving full details for the crystal data and data
collection parameters, atomic positional parameters,
anisotropic thermal parameters, bond distances and
bond angles for compounds 2 and 3 are available on
request from the author (R.A.W.).
Fig. 2. ORTEP [24] representation of the structure of the dirheniu-
m(III,II) complex Re2Cl5(Ph2PCH2CO2Me)3 (3). Thermal ellipsoids
˚
are drawn at the 50% probability level. Selected bond distances (A) and
bond angles (8) are as follows: Re(1)Ã
2.3324(15), Re(1)ÃCl(12) 2.3442(14), Re(1)Ã
P(2) 2.4827(16), Re(2)ÃCl(22) 2.3268(15), Re(2)Ã
Re(2)ÃCl(21) 2.3640(15), Re(2)Ã
1.205(9), C(12)ÃO(13) 1.326(9); Cl(11)Ã
P(1)ÃRe(1)ÃP(2) 151.79(5), Cl(22)ÃRe(2)ÃCl(23) 143.24(5), Cl(21)Ã
Re(2)ÃP(3) 153.55(5).
/
Re(2) 2.2318(3), Re(1)Ã
/
Cl(11)
/
/
P(1) 2.4532(15), Re(1)Ã
/
/
/
Cl(23) 2.3328(15),
Acknowledgements
/
/
P(3) 2.4654(16), C(12)Ã
/
O(12)
/
/
Re(1)ÃCl(12) 142.65(5),
/
We thank the John A. Leighty Endowment Fund for
support of this work.
/
/
/
/
/
/

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